Lithographic plate and process for making same



Patented Sept. 10, 1946 LITHOGRAPHIO PLATE AND PROCESS FOR MAKING SAME John D. Pursell, St. Louis, M0.

N Drawing. Application January 21, 1944, Serial No. 519,219

I (01. 955..6l (Granted under the act of March 3, 1883, as

2 Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, Without payment to me of any royalty thereon.

This invention relates to intaglio lithographic plates and more particularly to a process for making them.

Photolithographic printing plates are commonly formed on photographic film having a grained surface coated with a light sensitive emulsion. In preparing such plates for printin the light sensitive emulsion is exposed to light from either a positive line or half-tone copy or a negative of the image it is desired to print. The light ailected portion of the film is rendered insoluble and the remaining soluble portion of the film is removed either by chemical action or by washing with warm water to provide an intaglio printing surface. The-plates prepared by these processes have not been satisfactory because of their lack of durability and the lack of strength of portions of the printing surface of the film.

It is an object of this invention to provide a new and improved lithographic printing plate having a durable printing surface that will give a large number of impressions before losing its detail.

amended April 30, 1928; 370 O. G. 757) Another object or this invention is to provide vmetallic compound uniformly deposited in the non-image lnk-repe1ling surface of the plate.

A further object of this invention is to provide an inexpensive lithographic printing plate which is simple to prepare and which produces a large number of clear, detailed impressions.

Other objects will be apparent from the following description of. the invention.

These objects are accomplished by the following invention which comprises impregnating the non-image, ink-repelling gelatin surface of a lithographic printing plate with an aqueous solution of a soluble metallic compound which, thus introduced into the surface of the plate, is converted to an insoluble metallic compound either by the action of a reducing agent suspended in the gelatin surface such as the light-alfected particles suspended therein or by treatment with an aqueous solution of a reducing agent, as for example a solution of a developer. The insoluble metallic compound is thus uniformly deposited throughout the non-image, ink-repelling gelatin surface of the plate. The gelatin surface of the plate is hardened by the insoluble metallic compound deposited therein and the capacity of the surface to absorb moisture and lithographic solutions is greatly increased.

In practicing this invention, the lithographic printing plate may be prepared from any stand ard photographic film, though it is preferred to use the commercial photographic film known as Wash-off film. This film consists of a cellulose derivative or plastic base which is grained on one surface and coated with a light-sensitive gelatin emulsion on the grained side of the base. The light sensitive photographic film is exposed through its base to light from a positive line or half-tone copy either in a camera or by contact or projection; In obtaining photographic continuous tone copy a half-tone screen or mezzograph plate is placed before the film during exposure in the customary manner. The exposed film is treated with a solution of a standard tan-e. ning developer at room temperature. A suitable solution is as follows:

Dissolve 150 grams of sodium sulphate in 750 cc, of hot water and bring the volume of the solution to one liter by adding cold water. To this i solution add o-Dihydroxybenzene (pyrocatechiri) grn 8 Potassium bromide, 10% solution cc 10 Sodium hydroxide, 40% solution cc 25 This treatment not only develops the light-affected particles suspended in the emulsion but also tans, hardens and renders insoluble the gel atin emulsion adjacent the light-affected particles. Since standard tanning developers are difficult to keep due to their rapid oxidizing properties, the exposed film may be treated in an aqueous solution of any standard photographic developer and. subsequently treated with a tanning solution. A suitable tanning solution is as follows:

Parts Ammonium bichromate, 10% solution 1 Sodium chloride, 5% solution 1 Water 4 The developed film is immersed in the above solution for approximately 5 minutes.

The developed and tanned negative is rinsed and then agitated for about one minute in a 20 per cent aqueous solution of ammonium thiocyanate or in water at from to degrees Fahrenheit to dissolve and remove the 'untanned soluble gelatin and the undeveloped light-sensitive particles suspended therein from the sup- 3 porting base. The film base is thus bared in all parts corresponding to the positive image or the black parts of the original copy. The gelatin negative image is then treated to increase its hardness and resistance to Wear during printing as well as to improve its capacity for absorbing and retaining moisture or lithographic solutions. This is accomplished by impregnating the gelatin emulsion with an aqueous solution of a metallic compound which may be converted to an insoluble metallic salt. The compounds found to be particularly satisfactory for this purpose are soluble salts of mercury such as mercuric chloride, which are characterized by the property of being convertable into insoluble compounds. The film base is immersed in an aqueous solution of mercuric chloride and, as the gelatin surface of the fihn becomes saturated with mercuric chloride, this compound reacts with the light-affected particles suspended in the negative image and is converted to insoluble mercurous chloride. The reaction continues until a uniformly distributed deposit of insoluble mercurous chloride and insoluble silver chloride is formed throughout the gelatin surface of the film base. The progress of the reaction is visible due to the change of the negative image from a dark to a light color. The film is then rinsed thoroughly and dried at a low temperature.

The exposed portion of the film base readily accepts ink for printing purposes whereas the negative gelatin image absorbs moisture and the usual lithographic damping or etching solutions and when thus moistened will repel ink from those parts corresponding to the white or non print portions of the original copy. The dry film may also be treated with lithographic developing ink in the customary manner and allowed to dry. The film is then immersed in lukewarm water permitting the gelatin surface to absorb moisture and swell slightly, and the lithographic developing ink removed from the gelatin surface by gentle friction with wet cotton. The lithographic ink is retained by the bare portion of the film base representing the printing image. The plate may be dried for future use or used immediately in lithographic printing operations.

The lithographic printing plates of this invention may be made manually instead of photographically. This may be accomplished by drawing, typing or otherwise printing in tusche, litho ink or other suitable material on cellulose or a cellulose derivative or a plastic base and then coating the printed surface with gelatin. When the gelatin coating has set, the plate is immersed in turpentine or a petroleum solvent which dissolves the image material but does not attack the gelatin. The dissolved image material and the gelatin immediately supported thereon may be removed by gentle friction leaving the base exposed in places representing the positive image or printing area surrounded by a gelatin stencil representing a non-image or non-printing area. The stencil is then immersed in an aqueous solution of mercuric chloride until the gelatin surface is saturated with the solution. The plate is removed from the solution and treated with a reducing agent such as a photographic developer or a solution of sodium sulphite acidulated with a few drops of sulphuric acid to convert the soluble mercuric chloride to insoluble mercurous chloride. A uniformly distributed deposit of insoluble mercurous chloride is formed throughout the non-image gelatin surface of the plate which hardens this surface and increases its resistance to wear during printing as well as improves its capacity for absorbing and. retaining moisture or lithographic solutions.

This invention affords a simple and economical method of producing lithographic printing plates the non-image, ink-repelling surface of which possesses greater hardness and resistance to printing wear and moisture-absorbing capacity than ever attained previously. It is apparent that many different embodiments of this invention may be made without departing from the spirit and scope thereof and it is not intended to be limited except as indicated in the appended claims.

Having thus described my invention, what I claim as new and wish to secure by Letters Patent is:

l. The process for the production of a photolithographic printing plate having a printing surface consisting of an ink receptive image portion and a moisture receptive, non-image portion comprising exposing a photographic film to light projected from a copy, developing and tanning the exposed film to render the light-affected, nonimage portion of the printing surface insoluble, washing the film to remove the soluble portion of the film and produce an ink receptive image portion, and impregnating the non-image portion with an aqueous solution of mercuric chloride and bringing said mercuric chloride into contact with the light-affected particles suspended in the non-image portion of the printing surface to convert said mercuric chloride to insoluble mercurous chloride and produce a uniformly distributed deposit of insoluble mercurous chloride throughout said non-image portion.

2. A lithographic printing plate comprising a base having a grained surface and a lithographic printing surface on the grained surface of said base, said printing surface consisting of an ink receptive image portion formed by the grained surface of said base and a moisture receptive, inkrepelling nonimage portion formed by a gelatin coating on said base, said gelatin coating having a uniformly distributed deposit of water-insoluble mercurous chloride suspended therein.

JOHN D. PURSELL. 

